The introduction of a 16:9 wide aspect ratio picture format into existing broadcast television standards (e.g., PAL, SECAM, NTSC) having a standard 4:3 aspect ratio has resulted in a so-called "letter-box" image display format. This format has been discussed, for example, in "Possible Improvements and Trends for Development with PAL", G. Holoch, lecture FKTG Jan. 17, 1989 in Berlin, Germany; "Future Television Systems", F. Muller-Romer, Fernseh-und Kinotechnik, Vol. 43, issue No. 6/1989; and in "New Ways of the Old PAL", Dr. A. Ziemer, et al., Funkschau issue No. 18/1989. With the letterbox format, the total picture information from an original 16:9 widescreen source is displayed by a 4:3 receiver compatibly, with bars along the upper and lower edges of an image display exhibiting a 16:9 aspect ratio. The bar regions typically do not contain visible image information.
Letterbox coding at the studio involves, for example, removing every fourth line of a 575 active line image frame of a 16:9 widescreen image with, for example, 625 total lines. Certain lines are transmitted as additional information in the bar regions that are produced above and below the active image region as a result of the line removal process. The active picture area in a 4:3 receiver display then contains 431 lines, and the edge bar regions each contain 72 lines. The letterbox format for displaying widescreen 16:9 material by a 4:3 receiver is well known and requires no adaptation by a viewer of a standard receiver display.
Because the ratio of the two picture formats corresponds to the ratio of the original number of lines to the reduced number of lines, geometrical distortion otherwise produced by removing image lines is compensated for. The 16:9 widescreen receiver processes the information contained in the two edge bar regions for increasing the number of active image lines back to the original number (575 in this example), thereby creating a widescreen 16:9 picture display covering the entire display area of a widescreen display device.
Removing one out of four image lines from 575 active image lines in a frame to produce a compatible letterbox signal format creates signal artifacts and distortions that are visible in a 4:3 receiver display as, for example, diagonal artifacts or distorted image motion. To reduce or eliminate such effects, the lines can be vertically interpolated at the transmitter. However, this usually leads to reduced vertical resolution in a 4:3 receiver display.
To prevent the image information content of the lines disposed in the edge bar regions from becoming disturbingly visible at a 4:3 receiver, the amplitude of the image information in the bar regions may be attenuated at the transmitter by about 13 dB, and expanded accordingly at a 16:9 receiver. However, this can lead to increased noise in a 16:9 receiver display and, thereby, to reduced picture quality compared with a standard 4:3 receiver display.
A system for digital coding and transmission of EQTV signals in ISDN networks is described in "Compatible HDTV Coding for Broadband ISDN", K. H. Tzou et al., IEEE Global Telecommunications Conference & Exhibition, Nov. 28, 1988. In that system, a reduced definition EQTV picture is extracted from a high definition image by means of a two dimensional discrete cosine transform (DCT), digitally coded, and transmitted in data compressed form. An auxiliary channel conveys additional information by means of which a high definition image can be reconstructed by a high definition ISDN receiver. Portions of a 16:9 image exceeding the 4:3 format are also transmitted in the auxiliary channel. A 4:3 EQTV receiver reproduces only a part of the 16:9 image.
In accordance with the principles of the present invention, it is herein recognized as desirable to provide a compatible transmission system for a 16:9 aspect ratio television signal using a standard television channel which supplies letterbox format images for display by 4:3 aspect ratio standard receivers, and widescreen 16:9 aspect ratio images for use by 16:9 receivers, in particular with vertical image resolution substantially corresponding to that of the original image and with a low noise characteristic.